1. Field of the Invention
The invention relates generally to the control of arrays of electrostatic actuators. In particular, the invention relates to the digital control of micro electromechanical arrays of optical elements.
2. Background Art
Micro electromechanical systems (MEMS) utilize fabrication techniques developed in the semiconductor integrated circuit industry to produce very small mechanical systems, typically formed in a silicon wafer. One application that has prompted much interest in MEMS technology is optical communication using wavelength division multiplexing (WDM), although the invention is not limited to this application. A WDM communication system transmits multiple optical signals of different wavelengths on a single optical fiber, each wavelength carrier having its own data signal impressed thereupon. It is desired to create complex optical networks in which nodes of the network switch signals in different directions according to the wavelength of the signal and without the necessity to convert the optical signal to electrical form to effect the switching. The most typical form of such a switch includes a wavelength demultiplexer, such as a diffraction grating, which spatially separates the WDM signal into multiple optical beams of respective optical wavelength. These beams are separately and independently switched to wavelength multiplexers associated with the output fibers to form output WDM signals. Thus, an optical switch is needed having a number of independently controllable optical switching elements at least equal to the number of WDM channels, a number being pushed to 100 and higher.
Such a MEMS optical switch by Solgaard et al. in U.S. Pat. No. 6,097,859 and by Tomlinson et al. in U.S. Pat. No. 5,960,133. Both references use an array of mechanically movable mirrors formed from a silicon wafer. Each of the mirrors in the array are electrostatically controlled by a capacitor formed between a bendable mechanical element bearing the mirror and a control electrode formed under the mechanical element. It is known to have an array of gimbaled mirrors formed generally in the plane of the wafer but independently tiltable about two orthogonally arranged pairs of torsion bars as controlled by multiple capacitors formed under the mirror and its gimbal frame.
Such a MEMS switch or other type of array of MEMS elements requires the fabrication of large arrays of electronic microactuators to control the movement of the mechanical elements. Voltages of approximately 100V are typically required. MEMS capacitors that are DC biased tend to exhibit a charging effect which eventually prevents further actuation. Accordingly, the driving signals are preferably bipolar (AC). Further, the MEMS electromechanical elements and microactuators suffer from variations in their manufacturing and in environmental effects. Accordingly, the control systems needs to be able to separately tune the multiple microactuators.
High-voltage amplifiers and DC-to-AC converters are commercially available, and a control system can be easily implemented on a computer to set and adjust the amplitude of the voltage that drives a single microactuator. However, such an approach is not feasible in a commercial implementation of a MEMS array having a large number of array elements because the size and cost of the control system quickly overwhelm the MEMS array, which itself can be implemented on a single silicon chip.
Furthermore, the number of I/O lines needs to be constrained despite the requirement that each of the microactuators is independently controllable.
It is highly desirable that large parts of the control system be implemented on the same structure as are the MEMS elements and with the same pitch and approximately the same size. Nonetheless, the control system must accommodate the requirement of a high-voltage driver switching voltages and powers significantly greater than those associated with typical silicon control circuitry.
An analog control system for an array of movable mechanical elements, such as tiltable mirrors, formed in a micro electromechanical systems (MEMS). The movable elements may form part of variable gap actuator capacitors, whereby electrostatic actuators move the mechanical elements.
In each cell of the array, a hold capacitor is associated with each actuator capacitor. Control circuitry determines the amount of charge or voltage on each hold capacitor, preferably by incrementing and decrementing charge according to either a new state configuration or correcting the position in a feedback loop probing the output of the MEMS cell.
The power signal for updating the hold capacitor charge may be supplied from a single source, preferably in analog form controlled by a digital to analog converter, to separate drive circuits associated with each cell. An address decoder enables only one of the drive circuits to pass the power signal to the hold capacitor.
Preferably, an inverter couples the hold capacitor to the actuator capacitor, thereby allowing the actuator capacitor to be driven by a bipolar signal having equal positive and negative components.
The microactuators and electrostatic drivers are preferably formed in a single bonded structure in two dimensional arrays having the same pitch.